In a vehicle equipped with electric power steering (xe2x80x9cEPSxe2x80x9d), the steering assist torque may be provided by an electric motor coupled to a steering column or shaft. Typically, when a condition that requires an immediate shutdown occurs in such an EPS system, the assist torque provided by the motor is abruptly commanded to zero, power is removed from the transistors that drive the motor, and the system is shut down. An immediate shutdown might be triggered, for example, if the torque sensor failed or its signal was lost.
When an EPS system is shut down, the steering system typically reverts to manual steering. If the system is shut down while the EPS motor is applying a low magnitude assist torque, the shutdown may be imperceptible to the driver. However, if the system is shut down while the EPS motor is applying a moderate or higher magnitude torque, the driver may feel an abruptly increased reaction torque in the steering wheel or hand-wheel due to the sudden reduction in assist torque. The abruptly increased reaction torque results when the stored energy in the tires and steering system is released as the assist torque from the motor is instantaneously removed. Typically, the instantaneous removal of the assist torque allows the stored energy in the system to back drive the motor at moderate to high velocity for a moment. The driver feels this motion as the abruptly increased reaction torque in the steering wheel.
The above described and other features are exemplified by the following Figures and Description in which an electric power steering system is disclosed that includes a motor and a soft-disable controller in signal communication with the motor for reducing the abruptly increased reaction torque felt by a driver when an assist torque is abruptly disabled.